(1) Field of the Invention
The invention relates to a method of producing a microwave filter which comprises a body of gyromagnetic material and a source of a pre-polarizing magnetic field whose resonant frequency is a predetermined function of the temperature.
Such filters provided with one or more spheres of a gyromagnetic material such as yttrium iron garnet (YIG) are used in the microwave devices for realizing bandpass and bandstop filters having a high Q-factor.
(2) Description of the Prior Art
U.S. Pat. No. 3,713,210 issued on January 30th, 1973 to James M. Schellenberg discloses a method of stabilizing the resonant frequency of a YIG filter having a permanent magnet which acts as a source of the pre-polarizing magnetic field.
In accordance with this method, a YIG sphere is disposed in the field of a permanent magnet and the change in the resonant frequency across a given temperature range is measured.
With this data and the knowledge of the variation of the anisotropic field with temperature a correctional resonant frequency f.sub.b is calculated such that the change of the prepolarizing magnetic field with temperature is eliminated by the change with temperature of the anisotropic field.
This correctional resonant frequency can be expressed as: ##EQU1##
In this expression f.sub.a1 is the resonant frequency at the temperature T.sub.1, .DELTA.f.sub.a is the change in the resonant frequency when the temperature changes from T.sub.1 to T.sub.2, and Ha1 and Ha2 are the values of the anisotropic field at temperature T.sub.1 and T.sub.2, respectively.
The following numerical example illustrates an extreme case starts from an YIG filter having a permanent magnet consisting of an aluminium-nickel-cobalt alloy having a high Curie point.
At a given temperature range of, for example, 20.degree.-65.degree. C., the change .DELTA.H.sub.a in the anisotropic field may be approximately 20 Oersted (Ha1 = 45 Oersted, Ha2 = 25 Oersted) and the change .DELTA.Ho of the field of the permanent magnet may amount to approximately 4 Oersted. The change .DELTA.f.sub.a in the resonant frequency which occurs may be 120 MHz depending on the orientation of the field of the permanent magnet in the crystal lattice of the YIG sphere. The second term in the right hand portion of the equation (1) may then become 275 MHz.
The correctional resonant frequency may thus deviate considerably from the initiated adjusted resonant frequency f.sub.a1. This renders it imperative to make several adjustments to obtain a temperature stabilisation at a predetermined resonant frequency.